The present invention relates to a heat-sensitive recording material, and more particularly to a heat-sensitive recording material which is excellent in sensitivity and image stability, generates little scum adhering to a thermal head (hereinafter referred to as "thermal head scum"), and causes little fogging due to heat built up in a thermal head (hereinafter referred to as "fogging").
Generally, heat-sensitive recording materials comprise a support having provided thereon a heat-sensitive recording layer comprising, as essential components, an electron-donating dye precursor which is generally colorless or pale-colored, and an electron-accepting developer. Upon heating the heat-sensitive recording material by a thermal head, a thermal pen, a laser beam, or the like, the dye precursor and the developer instantly react with each other to form color. Thus, recorded images are obtained thereon. Such heat-sensitive recording materials are disclosed in Japanese Pat. Appln. Kokoku (Post-Exam.) Nos. S. 43-4160 and S. 45-14039 and the like.
Such heat-sensitive recording materials have advantages, for example, that records can be obtained by a relatively simple apparatus. Thus, heat-sensitive recording materials are used in a wide variety of fields such as measuring recorders, facsimile machines, printers, terminals of computers, labels, ticket vending machines, and the like.
Heat-sensitive recording materials require to have, for example, the following principal characteristics: The recording materials have sufficient sensitivity. Images to be formed thereon have sufficient optical density and do not substantially cause discoloration with the lapse of time.
Recently, attempts have been made for the purpose of speed-up of recording. Therefore, there has been desired development of a heat-sensitive recording material which has high sensitivity so as to give recorded images having sufficient density even when printing is carried out with low energy in order to adapt such attempts.
Furthermore, as heat-sensitive recording materials have been used so widely, images to be formed thereon are required to cause little discoloration even when fatty materials such as fats and oils contained in hair treatment agents, hand creams, sweat, etc. adhere to the recording materials.
As described above, heat-sensitive recording materials have been required to have higher sensitivity and image stability.
In order to obtain recorded images having high density, various sensitizers are contained in a recording layer of recording materials. As sensitizers, for example, Japanese Pat. Appln. Kokoku No. S. 43-4160 discloses urea, phthalic anhydride, acetanilide, and the like. Japanese Pat. Appln. Kokoku No. S. 48-19231 discloses natural and synthesized waxes such as beeswax, carnauba wax, paraffin wax, Japanese Pat. Appln. Kokoku No. S. 49-17748 discloses salicylic acid, monobenzyl ester of phthalic acid, and the like.
Although sensitivity is improved by use of these sensitizers, other characteristics are often deteriorated. For example, a considerable amount of thermal head scum is generated and serious fogging is caused.
In order to improve image stability, developers of bisphenol sulfone type are proposed. Specifically, Japanese Pat. Appln. Kokai (Laid-Open) Nos. S. 60-208286 and S. 60-13852 disclose bis(3-allyl-4-hydroxyphenyl)sulfone and 4-hdyroxy-4'-isopropyloxy-diphenylsulfone as this type developers, respectively.
At present, 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as "BPA") is widely used as a developer. Compared with recording materials comprising BPA, those comprising the bisphenol sulfone type developer have much improved image stability but are not sufficient in sensitivity.